The separation between the Over-Pressure region and the Under=Pressure regions developed in a compartment fire (sometimes referred to as the smoke/air interface). The neutral plane can be seen quite clearly when thermal balance exists in the fire compartment. 3D firefighting techniques can assist to keep the NP as high as possible, which maximises visibility and makes conditions more bearable for entrapped occupants or firefighters. Maintaining the height of the neutral plane is a key principle in successful, safe and efficient compartment firefighting.